Inkjet printing units which do not use printing plates are increasingly used in printing plate-based printing presses, mainly printing presses working on the principle of offset printing, e.g., in web-fed rotary offset printing presses and sheet-fed printing presses. These plateless inkjet printing units are used particularly for individualizing printed products produced by offset printing, e.g., with barcodes, numbering or other markings. Inkjet printing units of this kind have at least one inkjet printing head which can be designed to work on the principles of a continuous inkjet, drop-on-demand inkjet, thermal inkjet, bubble inkjet, or any other inkjet principle.
The maximum printing speed of inkjet printing units is appreciably less than the maximum printing speed of offset printing units. As a result there are difficulties involved in inline printing on a substrate by offset printing and inkjet printing.
To increase the achievable printing speed of inkjet printing units, it is known on the one hand to use inkjet printing units having a plurality of inkjet printing heads, namely, a plurality of inkjet printing heads positioned transverse to the transporting direction of the substrate or transverse to the printing direction and on the other hand to use a plurality of inkjet printing heads in the transporting direction of the substrate or in the printing direction where the plurality of inkjet printing heads being arranged next to one another in the shape of an array or matrix.
Although the printing speed of inkjet printing processes can be increased in this way, it has not been possible thus far in a web-fed rotary offset printing press designed, e.g., as a newspaper printing press operating at printing speeds for plate-based offset printing of up to 15 m/s to print on a substrate inline with a plateless inkjet printing process because the maximum achievable printing speed in conventional inkjet printing processes is 5 m/s. Consequently, when printing on a substrate with a plate-based printing process and, inline with the plate-based printing process, with a plateless inkjet printing process, the print image applied by using the inkjet printing process is subject to distortion. Accordingly, there is a need for a method for printing on a substrate by means of which the substrate can be printed upon at high printing speeds of more than 5 m/s inline or simultaneously with a plate-based printing process and a plateless inkjet printing process, specifically without the risk that a print image to be applied using the inkjet printing process is subject to distortion. A similar problem occurs, for example, when laser printing is used as a plateless printing method.
It is an object of the present invention to provide a novel method for printing on a substrate. This and other objects and advantages are achieved by a method in accordance with the invention, in which the printing resolution of the plateless printing process is determined depending on a printing speed of the plate-based printing process. In accordance with the present invention, the printing resolution of the plateless printing process, i.e. the inkjet printing process, is determined depending on the printing speed of the plate-based printing process, i.e. the offset printing process. The higher the printing speed of the plate-based printing process compared to the maximum printing speed of the plateless printing process at which printing can be performed at maximum printing resolution, the lower the printing resolution of the plateless printing process.
Consequently, it becomes possible to operate the plateless printing process at a higher printing speed than the maximum printing speed for which it is specified without the risk of distortion of the print image to be applied using the plateless printing process.
In a preferred embodiment, a printing resolution of the plateless printing process considered transverse to the transporting direction of the substrate is independent of the printing speed of the plate-based printing process; however, a printing resolution of the plateless printing process considered in the transporting direction of the substrate is determined depending on the printing speed of the plate-based printing process.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.